The present invention relates to a method of making improved cemented carbide tools for shaping or otherwise working materials. The invention has particular application in making metal working tools, and specifically tools used in the manufacture of tubular casings and similar articles, such as two-piece beverage cans.
A two-piece can is made by a drawing and wall ironing process. In general, a two-piece can is made by stamping out metal discs from a metal plate. A metal “cup” is formed from the disk. The formed cups are pushed through a body-forming die comprising a plurality of annular rings, generally known as draw, redraw, and ironing rings, by a body-forming punch. The clearances between the body-forming punch and the plurality of rings become progressively smaller, so that the thickness of cup wall is reduced and the cup is elongated. This process is generally referred to as the ironing operation. It is a particularly demanding operation causing high wear on the tools and the operation is sensitive to the dimensional changes and lubrication conditions. Because of the tremendous volume of beverage cans manufactured each year, each slight improvement in the manufacturing process can result in tremendous savings.
Tools for imparting a desired shape, form, or finish to a material, such as dies, punches, and the like, must be characterized by extreme hardness, compressive strength and rigidity. This is particularly necessary when shaping metals or similar materials. Commercial material working tools for mass production must also be resistant to wear, erosion and chipping from repeated and continuous stress and abrasion. These tools must also be made from materials which can be designed and machined to close tolerances and maintain dimensional stability over a wide range of operating conditions.
It is known to make punches, dies, deep draw tooling and similar material working tools from a variety of materials, including metals, cemented carbide and conventional ceramics. These known materials all have certain undesirable limitations. When making tools for shaping metal articles, particularly tubular casings such as two-piece beverage cans, the problems of prior known materials becomes particularly significant.
In the 1980's a grade having only 3 wt-% binder and ultra fine grain size for tire cord drawing was introduced by Sandvik. It was later withdrawn due to the low strength and brittle behaviour leading to premature failures.
In a European project, Wireman, (reported by A. M. Massai et al, “Scientific and technological progress in the field of steel wire drawing”, Wire 6/1999), the conditions for drawing of tire cord were investigated. New cemented carbide grades were tested in the grain size range of 0.3-1 μm and a binder content of 0.3-5 wt-%. A hardness increase was achieved by reducing the binder content and decreasing the grain size of WC. According to published results, the grades did not completely satisfy the expectation on better performance, despite the high hardness achieved. The conclusion quotes: “The wear tests demonstrated that not only the hardness of the dies controls the die wear mechanism.”
According to the prior art, a possible way to achieve better performance in can manufacturing is the use of ceramic materials, e.g. whisker reinforced alumina or silicon nitride as disclosed in U.S. Pat. No. 5,095,730 and U.S. Pat. No. 5,396,788 respectively, but so far conventional cemented carbide seems to keep its position as the preferred material.
The present invention relates to the recent development of ultra fine grained cemented carbide.
During many years there has been an ongoing development of cemented carbide with finer and finer grain size. The extension of cemented carbide grain sizes into the ultra fine size range leads to a number of positive improvements regarding the wear processes.
Attrition wear (or grain loss volume) may be reduced by an order of magnitude by little more than halving the sintered grain size (in the absence of other wear processes), since grain volume is related to the cube of diameter.
Adhesive fracture is another dangerous kind of attrition wear, in which the separation of strongly welded tool-workmaterial interfaces can induce tensile cleavage within the underlying carbide. Ultra fine hardmetals can resist the onset of such fractures better than coarser ones due to their greater rupture strength.
Erosion/corrosion of the binder phase is said to be part of the wear mechanism in wire drawing and the deep drawing of beverage cans. In ultra fine cemented carbide, even though the content of binder is maintained or even increased compared to conventional cemented carbide, the smaller WC grain size leads to thinner binder films. Thus resistance to selective erosion of the soft binder phase by wear particles is reduced. It is reasonable to believe that the thinner binder also leads to better oxidation/corrosion properties since the properties of the binder at the WC interface is different from the pure metal.
From the above it seems that the main interest in developing finer sub-micron hardmetal, perhaps into the nanometer range, is to raise hardness, maximise attrition wear resistance and strength whilst as far as possible maintaining all other attributes at useful levels.
Thus improved wear resistance of cemented carbide is achieved by decreasing the tungsten carbide grain size to ultra fine and maintaining the binder content so that the hardness as is increased.